Traditionally, vaccines have been based on live attenuated or inactivated pathogens. These strategies are inefficient, however, largely because of the antigenic variability of pathogens (e.g., viruses). Several peptide vaccines that comprise antigenic peptides or peptide fragments of pathogens have been developed. Conserved peptide fragments are less likely to exhibit antigenic variability and can overcome some of the problems associated with traditional peptides. Accordingly, subunit vaccines have been developed, which target conserved regions of pathogens. Synthetic peptide vaccines tend to be poorly immunogenic, however. The poor immunogenicity of synthetic peptide vaccines may be attributed to the fact that although these types of vaccines induce humoral antibody responses, they are less likely to induce cell-mediated responses.
Several investigators have sought to improve the antigenicity of synthetic peptide vaccines. For example, Klein et al. describe the engineering of chimeric proteins that comprise an immunogenic region of a protein from a first antigen linked to an immunogenic region from a second pathogen. (See, U.S. Pat. Nos. 6,033,668; 6,017,539; 5,998,169; and 5,968,776). Others have sought to create chimeric proteins that couple B-cell epitopes to universal T-cell epitopes in order to improve the immune response. (See, e.g., U.S. Pat. No. 5,114,713). Russell-Jones et al. (U.S. Pat. No. 5,928,644) also disclose T-cell epitopes derived from the TraT protein of Escherichia coli, which are used to produce hybrid molecules so as to generate an immune response to parasites, soluble factors (e.g., LSH) and viruses. Further, Ruslan (U.S. Patent Application Publication No. 20030232055) discloses the manufacture of vaccines based on PAMPs and immunogenic antigens.
The hepatitis B virus core antigen (HBcAg) is thought to be a key target for the host immune response in the control of the infection. In particular, the presence of HBcAg-specific T cells has been associated with clearance of acute and chronic infections with the hepatitis B virus (HBV). Subsequently, prophylactic and therapeutic vaccines that induce HBcAg-specific T cells have been developed and some have shown efficacy in infectious models. However, despite the high immunogenicity of exogenous HBcAg, many of the studies using endogenous HBcAg as a vaccine have been disappointing.
When expressed alone, HBcAg will spontaneously assemble into virus-like particles (VLPs) that are immunogenic in vivo. These VLPs interact with B cells as the primary antigen-presenting cell (APC) by an unusual interaction with the B cell receptor. HBcAg effectively primes specific T helper (Th) and, much less effectively, cytotoxic T cells (CTLs) as an exogenous antigen when high antigen doses in adjuvant are used. Both DNA- and retrovirus-based immunizations using HBcAg have been reported to induce detectable HBcAg-specific CTLs in mice. Some investigators have sought to use HBcAg VLPs as a platform to display heterogeneous antigens, as well, but these approaches have been hindered by poor assembly and instability of the particles. (See e.g., U.S. Pat. Nos. 4,818,527; 4,882,145; 5,143,726; 6,231,864; 6,887,464; 6,942,866; 7,144,712; 7,320,795; 7,351,413; and 7,361,352; the disclosures of which are hereby expressly incorporated by reference in their entireties).
DNA vaccines can be used as a model to study the endogenous immunogenicity of antigens. However, phase I/II clinical trials reveal that it is difficult to prime robust immune responses in humans with direct intramuscular injections of DNA vaccines. Different modes of DNA delivery have now become available, including transdermal delivery of DNA coated to gold beads using the gene gun or treatment of the injection site by in vivo electroporation. The need for approaches that enhance the immune response of a subject after vaccination, in particular DNA vaccination, is manifest.